Electrolytic method and means for production of refractory metal



Sept. 5, 1961 w. GENDVIL ETAL 2,999,055

ELECTROLYTIC METHOD AND MEANS FOR PRODUCTION OF REFRACTORY METAL Filed July 17, 1958 ZrCI Fig. l

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l l l l INVENTORS Leonard W. Gendvil BY Oliver W. Moles 710M 1344mm AGENT 2,999,055" Patented Sept. 5, 1961 2,999,055 ELECTROLYTIC METHOD AND MEANS FOR PRODUCTION OF REFRACTORY METAL Leonard W. Gendvil, Staten Island, N.Y., and Oliver W.

Moles, 'Watchung, N.J., assignors. to National Lead Company, New York, N.Y., a corporation of New Jersey Filed July 17, 1958, Ser. No. 749,280

2 Claims. '(Cl. 204--64) This invention relates in general to the electrolytic production of refractory metals. More particularly, it relates to the production of high purity zirconium metal by an electrolytic process adapted specifically to the production of zirconium metal on a commercial scale.

There are many compounds of zirconium which are found in nature but which are extremely difii-cult to reduce to the metallic state. In many instances it is relatively easy to convert such compounds to the halides of zirconium by halogenation processes but it is difiicult to produce zirconium metal from zirconium halides.

Zirconium metal has heretofore been'produced from its compounds by thermal reduction methods or by direct chemical reduction of the halide employing metallic sodium or metallic magnesium as reducing agents. Such methods for producing zirconium metal are described in the literature, for example, in the process of Lely and Hamburger in Zeitschrift anorganische Chemie, voltime 87, pages 223 to 225 in 1914, and in the Kroll process in the Transactions of the Electrochemical Society, volume 89, pages 263 to 274 in 1946. While these methods have produced metallic zirconium, they suffer from a fundamental economic disadvantage in that the cost of the reducing metal employed, such as sodium or magnesium, is relatively high and the processes are expensive and cumbersome to operate.

The production of zirconium metal by a direct electrolytic process has now been accomplished, as described and claimed in the instant application by which it is now possible to produce zirconiummetal of high purity on a commercial scale.

An object, therefore, of the present invention is to provide an electrolytic process for producing zirconium metal of high purity from zirconium metal halides.

A further object is to provide an improved method for operating an electrolytic cell whereby zirconium metal of high purity may be produced economically in relatively large quantities and in a semi-continuous commercially feasible manner.

Another object of -the invention is to operate an electrolytic cell of the fused salt bath type in such a manner as to deposit zirconium metal from said bathonto a porous cathodic surface, meanwhile maintaining the zirconium metal deposit perforative so as to recover substantially all of the zirconium metal in the form of relatively large coarse particles.

A still further object of theinvention is to provide a superior electrolytic cell for .producing' highly ductile zirconium metal on a commercial scale.

These and other objects of this invention will be come apparent from the following more complete decript-ion thereof and the accompanying drawings 3 in which:

FIGURE 1 is a vertical cross-section of an electrolytic cell embodying a perforated cathodic surface inthe form of a cathodic basket-like member; and

FIGURE 2 is a plan view of the cell on section line electrolytic cell haviuga cathode, an anode and 'electrolyte, said electrolyte being a fusedsalt bath and said cathode being a basket-like member having an impervious top-section and a perforated body-section, which comprises; introducing said basket-like cathode beneath the surface of said fused salt bath, introducing an inert gas and vaporous zirconium tetrachloride into the impervious top-section of the cathodic basket, maintaining sufiicient pressure in the impervious top-section of said cathodic basket to displace the fused salt therefrom and provide a gas zone in saidimpervious top-section into which gas zone the inert gas and gaseous zirconium tetrachloride are introduced, and during the addition of the inert gas and zirconium tetrachloride simultaneously passing direct current between the anode and the cathode at a rate synchronized with the zirconium tetrachloride addition so that the amount of current is suflicient to reduce the zirconium metal so formed as an adherent pervious mass of crystalline zirconium metal on the interior surfaces of thevperforated body-section of the cathodic basket,

the electrolyte eXteriorly of the cathodic basket-likemember being maintained substantially free from zirconium values.

In general the electrolytic reduction of the halides of refractory metals to form a refractory metal on a cathode by passing direct current through a cell at a rate synchronized with the rate of addition of the refractory metal halide to the fused salt bath so that the amount of current is sufficient to reduce a substantial portion, if not all of the metal halide substantially directly. to metal, is known and hereinafter referred to as direct electrolysis.

According to the present invention it has been discovered that the production of refractory metals, and in particular zirconium metal, by direct electrolysis may be considerably improved not only by confining the zirconium metal halide in the fused salt bath to the immediate vicinity of a cathodic surface such that the zirconium metal will be deposited substantially in toto thereon but that by controlling the operating conditions of aparticular cell and the type and orientation of the cathodic surface relative to the anode and the point of introduction of the zirconium halide into the fused salt bath, the zirconium metal deposit may be maintained perforative for a period of time sufiicient to insure the practical commercial operation of the cell and",

bath is not permitted to diffuse throughout the molten salt electrolyte but is directed toward and maintained substantially in intimate contact with a cathodic surface whereby the zirconium halide values are reduced to metal in the formof a zirconium metal deposit on the cathodic surface. The preferred method and means by which the zirconium halide may be confined to the immediate vicinity of a cathodic surface and the metal deposit formed thereon forms the subject matter of the instant invention the cathode current density, current-to-ZrCL feed ratio, r

, form.

and is described hereinafter in detail.

The phrase operating conditions, as used herein, has reference to the control which must be maintained over variables such as time, temperature of the fused salt bath,

reduction of zirconium halide values to zirconium metal explained'that by perl as large deposits of ductile crystalline metal can be accomplished on a commercial scale by the use of a perforated cathodic surface, preferably in the form of a basket, into which the ZrCl is introduced above the surface of the electrolyte. It may be postulated that ionic currents proceed from the anode through the perforations in the walls of the basket and through the voids in the perforative deposit of zirconium metal to the inside of the basket where zirconium ions may he found and reduced to zirconium metal on the inner walls thereof. Contrary to what one might expect, a major portion of the ionic current does not stop at the exterior surfaces of the cathodic basket but apparently continues on through the perforations of the basket wall and through the relatively infinite number of substantially parallel paths in the perforative deposit of zirconium metal to the interior of the basket.

By utilizing the process of the instant invention, zirconium metal of high purity and ductility may be obtained in the form of a deposit of coarse crystalline porous metal inside the body-section of the basket-like cathode below the gas zone.

In order to describe the process of the instant invention in more detail, the following description is presented.

Referring to FIGURE 1, the apparatus shown consists of a cell container 10 heated by graphite electrodes 11-11. The cell container is filled or partially filled with a fused salt electrolyte 12 in which is suspended a pair of graphite anodes 1313 between which is supported a cathode member indicated generally at 14, The fused salt electrolyte, sometimes referred to as a fused salt bath, comprises preferably a molten halide salt of an alkali or alkaline earth metal, including magnesium, particularly the chlorides of said metals which may be employed singly or in combinations. Mixtures of these halides which form low melting point eutectics are most convenient to employ such as, for example, mixtures of sodium chloride and strontium chloride, sodium chloride and lithium chloride, sodium chloride and barium chloride, sodium chloride and magnesium chloride or mix: tures thereof. In general, the temperature of the fused salt bath may vary within the range of from 375 C. to 950 C. depending upon the particular salts used, the type of metal being deposited and the construction of the cell itself. For the production of zirconium metal using a sodium chloride bath, operating temperatures within the range of from 825 C.-950 C. have been found suitable.

Referring again to the cathode member 1d, the latter comprises a metal feed pipe 15 for introducing the inert gas and the zirconium tetrachloride into the electrolyte; and a substantially rectangular cathodic enlargement adjacent its lower end comprising preferably a metal basketlike member having perforated side walls 16, hereinbefore referred to as a perforate body-section, and an imperforate top-section 17. The basket is formed of zirconium metal although sheet iron or steel may be used, and inasmuch as it is integral with or connected electrically to the cathodic feed pipe 15 by welded metal or the like, it itself is cathodic. An important control in the operation of the cell is that of introducing the incoming zirconium tetrachloride and the inert gas into the interior of the basket-like member 14 and in particular, in a gas zone 19 in the imperforate top-section 17 for it has been discovered that if a gas zone is not maintained in the impervious top-section of the cathodic basket, the zirconium tetrachloride will dissolve so rapidly in the fused salt that the salt will rise up in the gas introduction tube 15 and plug the feed system. However, if a gas zone is maintained between the fused salt bath and the open end of the gas introduction tube no plugging dilficulties .occur.'

Toy this, end the feed pipe 15 terminates at its point of fused salt from the imperforate top-section of the basket and creating a gas zone 19 therein. The fused salt bath inside the basket is therefore confined to the perforated body-section 16 of the basket i.e. below the gas zone 19 and hence away from the end of the introduction tube 15. As a, consequence, the gaseous zirconium tetrachloride first enters the gas zone 19 of the basket mthode and then is dissolved in the salt bath within the basket below the gas zone thus precluding any plugging of the intro duction feed pipe 15. With the application of an electric current to the cell the dissolved zirconium tetrachloride is reduced to metal and deposited on the inside walls of the perforated body-section of the basket-like cathode.

Other variables which must be controlled in the operation of any given cell so as to insure the formation of a perforative deposit of zirconium metal are the size and arrangement of the holes in the walls of the basket and the basket thickness. Although holes varying from /s" in diameter spaced 4" on centers (16 holes to the square inch), to holes /2" in diameter spaced 1 on centers (1 hole per square inch) have been used, the preferred arrangement is A" holes spaced /z on centers (4 holes to the square inch). Moreover, cathodic baskets having body-sections comprising two or four perforated sides are satisfactory but since a rectangular basket having a bodysection comprising two oppositely disposed perforated sides provides relatively constant plating area, this construction is preferred.

For reduction to take place on the inside or inner surfaces of the zirconium deposit it is necessary that the ionic current pass through the perforative deposit of zirconium metal to the interior of the basket where the zirconium ions are found. As the deposit increases in thickness it offers increased resistance to the passage of the ionic current which for constant rate of reduction requires higher voltage and amperage. This in turn results in lower current etficiencies. In brief, the nature of the basket operation is such that it will cut itself off. However, if operated under the conditions set out herein, a deposit of practical thickness and porosity will be formed on the inner walls of the basket.

In carrying out the process of the instant invention the zirconium halide is added preferably in vapor form with the inert gas into the gas zone in the impervious top section of the cathodic-basket, the gas mixture being added at a rate and at a pressure sufiicient to maintain the gas zone in the impervious top-section. The zirconium tetrachloride vapor is thus rapidly dissolved into the fused salt bath.

In addition to maintaining the gas zone in the imperforate top-section of the basket, it is also necessary to pass concurrently a predetermined amount of electric current through the cell at a rate synchronized with the rate of zirconium tetrachloride addition in order to reduce substantially immediately the zirconium tetrachloride to zirconium metal and thereby prevent the zirconium values in the salt bath from passing through the perforated walls of the cathodic basket toward the anode where they would not be reduced to metal.

A theoretically suificient current will comprise four faradays of electricity passed concurrently through the cell while one mole of zirconium tetrachloride is being introduced into the cell. In actual practice, however, it has been found desirableto add a quantity of electricity somewhat in excess of the theoretical amountin order to make up the current loss caused by side reactions in the cell. This extra quantity of electricity will vary depending upon the cell design. With the types of cell shown in FIG. 1, it has been found desirable to add from about 4.5 to 6.0 faradays of electricity per mole of zirconium tetrachloride introduced in order to maintain eflicient cell operation.

Theoretically, if less than one mole of the zirconium. tetrachloride is introduced. into the .cell for. each four faradays of electricity which pass through the cell, other added in a quantity inexcessof one mole for each'four faradays of current which pass through the cell, zirco-.

nium tetrachloride or .lower valent zirconiumchlorides will be established in the electrolyte and will diffuse and be transferred through the path to the anode where the lower valent zirconium will be reoxidiz ed. The,efliciency of such an operation will be noticeably. decreased when the amount of zirconium tetrachloride exceeds about 0.9 mole of zirconium tetrachloride introduced for each four faradays of electricity. I

An additional factor in the operation of the cell is the cathode current density which has been defined as current per unit area of perforated cathodic surface, the area" being the multiple of the linear dimensions of the surface. In general, a cell of the type shown is operated given cell to produce a perforative deposit of zirconium metal is the interdependency of feed rates, current tofeed ratio and current density. As seen in the table below, runs of 48 hours duration were made using feed rates varying from 0.96 to 1.74 lbs./hr./ft. current-tofeed ratios of from 4.5 to 7.0 faradays per mole and current densities of from 200 to 500 amperes per square foot. Generally at constant current-to-feed ratio the deposit becomes denser with increasing current density while with constant current density the deposit decreases in density with increasing current-to-feed ratio. The table below shows approximate ranges of conditions determined by actual runs as being satisfactory for the deposition of perforative deposits of zirconium metal on aperforated cathodic surface the preferred operating range being within the enclosed area as shown in the table. Too high a feed rate has been found to result in either zirconium ions escaping from the basket, in which case feeding of the ZrCl, must be stopped because of low deposition rates, or plugging of the; deposit. On the other hand, too low a rate results in inefiicient utilization of equipment.

TABLE [Feed Rates (ii/hL/ftfl) for operating cell over 48 hour eriod at various current-to-feed ratios andeathode current densities ased on area of perforated sides)] Cathode current density in Currenbto-leed ratio in Faradays/ amperes/square foot mole ZrClr 4.5 .i .l.. i .85

In order to more fully illustrate the instant invention, the following examples are presented:

Example 1 Using a cell of the type shown in FIG. 1, a basket-type cathode having an imperforate top-section, a body-section pounds of 45% barium chloride. and 55% sodiumcomprising two opposed perforated sides provided with %'f apertures spaced on centers, two solid sides and; a solid bottom and having a /2" feed pipe for the intro chloride maintained at a temperature of about 730 C. Zirconium tetrachloride vapors were then introduced into the top section at the rate of 2.83 lbs. per hour along with helium gas at the rate of 10 cubic centimeters (STP) per min. through the feed pipe into the basket cathode. The perforated walls of the body-section of the basket were arranged below the irnperforate top-section and opposite the respective anodes of the cell and served to confine the zirconium values within the cathodic basket. At this rate of gas addition, a pressure of about 0.6 pound per square inch was maintained in the gas zone in the imperforate top-section of the basket, thus forming a layer of helium gas above the salt bath containing zirconium ions. Concurrently, an electric current equivalent to 5.4 faradays per mole of zirconium tetrachloride introduced was passed through the cell. This amount of currentwasin effect sufficient to completely reduce the zirconium tetrachloride to zirconium metal substantially all of which was deposited on the inner perforate walls of the basket as relatively coarse particles of metal. In order to obtain substantially 5.4 faradays per mole of zirconium tetrachloride introduced,

800 amperes with an impressed voltage of approximately 5.66.5 volts .was required. The cathode current density was about 400 amperes per square foot. The run was made for a period of 29 hours, during which time the deposit of zirconium metal remained perforative and the.

apertures in the body-section of the basket open. The basket cathode was withdrawn from the cell and zirconium metal was found deposited on the perforated walls conium tetrachloride was recovered as zirconium metal Example 11 Using a cell of the type shown in FIGURE 1, a basket type cathode having an impenforate top-section and pro vided with a body-section comprising two perforated sides having A" apertures spaced 2 on centers, two,

solid sides and a solid bottom; and having ;a /2" feed pipe for introducing zirconium tetrachloridevapors into the basket was lowered into a fused salt electrolyte consisting of 600 lbs. of sodium chloride maintained at a temperature of about 850- C. Zirconium tetrachloride vapors and helium gas were introduced into the gas zone within the imperforatetop scction at the rate of 975 grams per hour for the zirconium tetrachloride and 10 cubic centimeters per minute of helium gas. Concurrently, an electric current equivalent of 7.1 faradays per mole of zirconium tetrachloride introduced was passed through the cell. This amount of current was, in eifect, sufiicient to completely reduce the zirconium tetrachloride to zirconium metal. In order to obtain substantially 7.1 faradays per mole of zirconium tetrachloride introduced, 800 amperes with an impressed voltage of approximately 4.8-5 .7 volts was required. The cathode current density was about 500 amperes per square foot. The run was made for a period of 47 hours during which time the deposit of zirconium metal in the perforated body-section of the basket remained perforative and the apertures in the walls of the basket open. The basket cathode was withdrawn from the cell and the zirconium metal was found deposited on the interior perforated walls of the basket cathode in theform of an irregular perforative tenacious mass composed of relatively large crystals. The leached zirconium metal recovered, as described in the preceding example, weighed 39.5 pounds which analyzed substantially 100% zirconium and possessed a Brinell hardness of about 198.

Example 111' Using the cell shown in FIGURE 1 and a basket cathode having an imperforate top-section and having a body-section provided with one perforated side having apertures Ma" in diameter and A on centers, three solid sides and a solid bottom, and a /2" feed pipe, zirconium tetrachloride was'fed into the gas zone within the imperforate top-section of the basket above the electrolyte therein at the rate of 910 grams per hour and the bath was maintained at a temperature of about 850 C. Helium gas was also introduced with the zirconium tetrachloride to maintain a layer of helium gas within the gas zone within the top-section of the basket. An electric current equivalent to 5.7 faradays per mole of zirconium tetrachloride was passed through the cell, the amount of current required being 600 amperes at an impressed voltage of from 4.5 to 4.9 volts. The cathode current density was about 300 amperes per square foot and the 11111 was continued for 52 hours. The zirconium metal produced by this run comprised 33.3 pounds of coarse material which represented 81.6% yield of zirconium and had a Brinell hardness of 278.

It has been clearly shown by the description of the instant invention and by the examples presented that zirconium metal may be obtained by passing zirconium tetrachloride into a basket-type cathode in an electrolytic cell at a rate synchronized with the electric current addition such that the amount of electricity added per mole of zirconium tetrachloride measured in faradays is numerically substantially greater than the number of chloride atoms present in the said zirconium tetrachloride molecule; and that by confining the zirconium chloride molecules to the immediate vicinity of the cathode while concurrently retaining the zirconium metal deposit on the cathodic surfaces of the basket, high recoveries of highly ductile, relatively coarse zirconium metal may be obtained. The invention is further characterized by the use of simple and inexpensive apparatus whereby it is possible to produce zirconium metal economically and continuously.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto, and other modifications and variations may be employed within the scope of the following claims.

We claim:

1. A method for producing zirconium metal in an electrolytic cell having a cathode, an anode and an electrolyte; said electrolyte being a fused salt bath and said cathode being a basket-like member having an impervious topsection and a perforated body-section, which comprises;

immersing the basket cathode in said fused salt bath, introducing an inert gas and vaporous zirconium tetrachlorideinto the impervious top-section of said basket cathode above the surface of the salt bath in said basket cathode, maintaining sufliicient gas pressure in the impervious top section of said basket cathode to displace the fused salt therefrom and pnovide a gas zone in said top-section in which gas zone said inert gas and gaseous zirconium tetrachloride are contained, said gaseous zirconium tetrachloride being dissolved in said fused salt bath from said gas zone, simultaneously passing direct current between the anode and said basket cathode at a rate synchronized with the zirconium tetrachloride addition so that the amount of current is sufficient to reduce the zirconium tetrachloride to metal; and depositing the zirconium metal so formed as an adherent pervious mass of crystalline zirconium metal on the interior surfaces of the perforated body'section of said basket cathodic member, the electrolyte exteriorly of the basket cathode being maintained substantially free from zirconium values.

2. A method for producing zirconium metal in an electrolytic cell having a cathode, an anode and an electrolyte; said electrolyte being a fused salt bath and said cathode being a basket-like member having an impervious top-section and a perforated body-section which comprises; immersing the basket cathode in said fused salt bath, introducing an inert gas and vaporous zirconium tetrachloride into the impervious top-section of said basket cathode above the surface of the salt bath in said basket cathode, maintaining sufficient gas pressure in the impervious top section of said basket cathode to displace the fused salt therefrom and provide a gas zone in said top-section in which gas zone said inert gas and gaseous zirconium tetrachloride are contained, said gaseous zirconium tetrachloride being dissolved in said fused salt bath from said gas zone, simultaneously passing direct current between the anode maintaining said basket cathode at a rate synchronized with the zirconium tetrachloride addition so that the amount of current is sufficient to reduce the zirconium tetrachloride to metal; and the current-to-feed ratio within the range of from 4.5 to 7 faradays per mole, the cathode current density within the range of 200 to 500 amperes per square foot and the feed rate of said zirconium tetrachloride being in the range of from 0.96 to 1.74 pounds per hour per square foot such that said adherent mass of zirconium metal is perforative and the electrolyte exteriorly of the cathodic basket-like member is substantially free from reduced zirconium values.

References Cited in the file of this patent UNITED STATES PATENTS 2,749,295 Svanstrom et a1 June 5, 1956 2,801,964 Opie et a1. Aug. 6, 1957 2,900,318 Andrews Aug. 18, 1959 FOREIGN PATENTS 160,930 Australia July 19, 1951 

1. A METHOD FOR PRODUCING ZIRCONIUM METAL IN AN ELECTROLYTIC CELL HAVING A CATHODE, AN ANODE AND AN ELECTROLYTE, SAID ELECTROLYTE BEING A FUSED SALT BATH AND SAID CATHODE BEING A BASKET-LIKE MEMBER HAVING AN IMPERVIOUS TOPSECTION AND A PERFORATED BODY-SECTION, WHICH COMPRISES, IMMERSING THE BASKET CATHODE IN SAID FUSED SALT BATH, INTRODUCING AN INERT GAS AND VAPOROUS ZIRCONIUM TETRACHLORIDE INTO THE IMPERVIOUS TOP-SECTION OF SAID BASKET CATHODE ABOVE THE SURFACE OF THE SALT BATH IN SAID BASKET CATHODE, MAINTAINING SUFFICIENT GAS PRESSURE IN THE IMPERVIOUS TOP SECTION OF SAID BASKET CATHODE TO DISPLACE THE FUSED SALT THEREFROM AND PROVIDE A GAS ZONE IN SAID TOP-SECTION IN WHICH GAS ZONE SAID INERT GAS AND GASEOUS ZIRCONIUM TETRACHLORIDE ARE CONTAINED, SAID GASEOUS ZIRCONIUM TETRACHLORIDE BEING DISSOLVED IN SAID FUSED SALT BATH FROM SAID GAS ZONE, SIMULTANEOUSLY PASSING DIRECT CURRENT BETWEEN THE ANODE AND SAID BASKET CATHODE AT A RATE SYNCHRONIZED WITH THE ZIRCONIUM TETRACHLORIDE ADDITION SO THAT THE AMOUNT OF CURRENT IS SUFFICIENT TO REDUCE THE ZIRCONIUM TETRACHLORIDE TO METAL, AND DEPOSITING THE ZIRCONIUM METAL SO FORMED AS AN ADHERENT PERVIOUS MASS OF CRYSTALLINE ZIRCONIUM METAL ON THE INTERIOR SURFACES OF THE PERFORATED BODY-SECTION OF SAID BASKET CATHODIC MEMBER, THE ELECTROLYTE EXTERIORLY OF THE BASKET CATHODE BEING MAINTAINED SUBSTANTIALLY FREE FROM ZIRCONIUM VALUES. 